1. Field of the Invention
The ivention relates to an active very-high frequency circuit of the all-pass type, comprising an amplifier stage and an RC network.
The invention finds its application in realizing image frequency rejection mixers intended, for example, for receiver front ends for signals relayed by artificial satellites or for microwave radio signals in general. The invention also finds its application in realizing modulators for four or more phases.
2. Discription of the Related Art
A phase-shift circuit is known from the publication entitled: "Monolithic RC All-Pass Networks with Constant-Phase-Difference Outputs" by Stephen K. ALTES et al. published in "IEEE Transactions on Microwave Theory and Techniques", VOL. MTT-34, No. 12, Dec. 1986, pp 1533-1537.
This document describes a phase-shift circuit realised on the basis of an RC network, comprising first of all two field effect transistors in a buffer arrangement for decreasing the output impedance of the stage preceding the RC network. Each of these transistors has its drain connected directly to a d.c. supply voltage, its source connected to ground through a resistor and has a considerable gate width (120 .mu.m). Each transistor receives at its gate an input signal having the same amplitude as that of the signal received by another transistor but having the opposite phase.
The source of each of the transistors is also connected to one of the ends of a network formed by four parallel branches. Each branch is constituted by a series RC network. The outputs of the phase-shift circuit are connected to the node of the capacitor and the resistor of each branch. The resistors and capacitors of each branch are provided such that each output has the same amplitude and a phase difference of 90.degree. relative to the next output. Furthermore, a switching circuit is provided for switching from one pair of outputs to another pair.
This circuit operates in the 220-280 MHz band, which frequency is much too low for the applications considered for this invention, these applications requiring a phase shifter operating at least in the 8-12 GHz band.
In the above publication a second circuit operating in the 3-5 GHz band is discussed. But this frequency domain is again too low for the applications under consideration. This result is achieved in this second circiut because it is a second-order circuit which is obtained by adding a certain number of components. A large number of components is still unfavourable for large scale integration which is searched for.
In addition, the second-order cirucuits have considerable insertion losses relative to first-order circuits. Moreover, its operation is based on the same principle as the above first-order circuit.
It is important to understand that in the prior-art circuits the load impedance of these circuits (or input impedance of the following circuit) appears in the transfer function so that the modulus (absolute value) of this transfer function depends on a time constant in which this impedance occurs, and on the frequency. The transfer function is thus that of a non-ideal all-pass function.
In order that the prior-art circuit(s) have a transfer function which is nearest possible to the ideal all-pass function, two conditions are to be fulfilled at the same time. First the output impedances of the buffered transistors are to be low with respect to the resistance of the RC network. Secondly, the input impedance of the next stage is to be high with respect to the impedance of the capacitor of the RC network.
Since the impedances of these preceding and following stages are fixed, the transfer function of the prior art network(s) can never be an ideal all-pass function, and this is more and more perceptible accordingly by as the frequency increases.
On the other hand, in each of the RC networks the values of the capacitor and resistor are fixed so as to obtain different time constants, allowing obtaining the desired phase shift on each path. From this it results first of all that the gain on each path is different. Then, if it is desired to increase the operating frequency, the result is that the RC products have to be diminished in order to diminish the time constants. Now we have seen hereinbefore that the resistor and capacitor values of the RC networks have to be maintained within certain boundaries imposed by the impendances of the preceding and following circuits, in order to remain as near to the ideal all-pass function as possible.
Under these conditions, the prior-art circuit(s) remains (remain) restricted to relatively low frequencies, or rather shows (show) very repidly degraded performance as regards amplitude and phase.
It is also important to understand that the prior-art circuit(s) always requires (require) two input signals having the same amplitude and opposite phase. On one hand the generation of these signals on the basis of a single signal requires the introduction of an additional circuit, which augments the surface of the network and its power consumption. On the other hand it is very difficult to obtain signals having exactly the same amplitude and opposite phase.